1. Field of the Invention
The present invention relates to a wireless receiver device and, more particularly, to a general receiver device with adaptive filter.
2. Description of Related Art
Typical receiver architectures can be divided into a single conversion with intermediate frequency (IF), a single conversion with low IF, a single conversion with zero IF, a dual conversion with IF, a dual conversion with low IF, and a dual conversion with zero IF. In the architectures of the single conversion with IF, the dual conversion with IF, and the dual conversion with low IF, an IF filter performs the frequency selection and filtering function. A surface acoustic wave (SAW) filter has the advantages of high quality factor (Q factor), low frequency offset, and low power consumption, but the SAW filter requires a special process and is difficult in integration into a single chip. Thus, a receiver is typically connected with an external SAW filter.
FIG. 1 schematically illustrates a typical tuner with a SAW filter, which converts a radio frequency (RF) signal into an intermediate frequency (IF) signal. The SAW filter 110 is disposed outside the tuner 120. Typically, a SAW filter has very high Q factor and better frequency selectivity, but a higher price makes the entire cost of a tuner significantly high. Also, the input impedance of a SAW filter is typically designed to be 50 ohms, so that the SAW filter driver 121 requires hundreds of mW in power consumption for driving the low impedance SAW filter externally connected. Further, the SAW filter itself has very high signal loss, which indicates a reduction in signal to noise ratio (SNR).
For replacing the SAW filter and integrating it into a single chip, a filter can be designed as various types such as a resistor-capacitor filter (RC filter), a switch-capacitor filter (SC filter), and a transconductor-capacitor filter (Gm-C filter).
The RC filter is applied only for a KHz-order filter in KHz due to the limited bandwidth of operational amplifier, and not suitable for an IF operation. For the SC filter, the parameters are set based on the capacitance ratio. In a CMOS process, the capacitance ratio can be controlled accurately. In this case, the SC filter can use the capacitance ratio to determine the filter characteristic without being easily affected by the process, but it is not suitable for a filter over 10 MHz due to the high power consumption and the bandwidth of operational amplifier.
The Gm-C filter uses one or more transconductance amplifiers and/or capacitors to simulate a resistive and inductive effect. FIG. 2 schematically illustrates a transconductance amplifier simulating the resistive effect in the prior art, where the simulated resistance is
      1          G      m        ,for Gm indicates the transconductance of the transconductance amplifier 210. FIG. 3 schematically illustrates two transconductance amplifiers and one capacitor which simulate the inductive effect in the prior art, where the simulated inductance is
      C                  G        1            ×              G        2              ,for C indicates the capacitance of the capacitor 310, G1 indicates the transconductance of the transconductance amplifier 320, and G2 indicates the transconductance of the transconductance amplifier 330.
For designing a Gm-C filter, the circuit of resistor-inductor-capacitor (RLC) filter has to be designed first. FIG. 4 shows a circuit of a typical RLC filter. In FIG. 4, the resistors can be replaced with the circuit of FIG. 2 while the inductors can be replaced with the circuit of FIG. 3. Therefore, when the Gm-C filters are used to design an IF filter, it is likely to cause the problem that the circuit area is too large. In addition, the coefficients of the Gm-C filters are typically a product of two different components. For example, as cited above, the simulated inductance is obtained by dividing the capacitance of the capacitor 310 by a product of the transconductance of the transconductance amplifiers 320, 330. Further, the error rate for the coefficients of an SC filter is as low as 0.1%, but the error rate for the coefficients of a Gm-C filter in the initial stage is as high as 30%.
As cited, the tuner in FIG. 1 has the disadvantages of over-high cost and complicated integration due to the use of SAW filter. When the Gm-C filter is used in the tuner for replacing the SAW filter, it has the disadvantages of over-large area and over-high cost. In addition, in many wireless receiver applications, such as a terrestrial digital video broadcasting (DVB-T), the central frequencies and the bandwidths are different. Unfortunately, with either the SAW or Gm-C filter, the central frequency of the tuner is fixed and thus cannot be adjusted according to the applications. Therefore, it is desirable to provide an improved receiver device to mitigate and/or obviate the aforementioned problems.